Wavefront aberration correcting device and optical pickup equipped with the same

ABSTRACT

An object of the present invention is to provide a wavefront aberration correcting device comprising a liquid crystal device that maximizes transmittance and minimizes light transmittance fluctuations even when the liquid crystal is driven by voltage. Disclosed is a wavefront aberration correcting device for correcting a wavefront aberration of light generated in an optical path of an optical system for irradiating light onto a recording medium or guiding the reflected light reflected by the abovementioned recording medium is characterized in that it comprises a pair of opposing transparent electrode layers provided in the abovementioned optical path; and a liquid crystal sandwiched between the abovementioned transparent electrode layers, for generating phase change in passing light due to voltage applied to the abovementioned transparent electrode layers, the abovementioned transparent electrode layer is arranged on an antireflective body comprising a substrate, and a finestructure formed on a substrate and having a concave-convex structure.

TECHNICAL FIELD

The present invention relates to an optical pickup device for thereading and writing of information on an optical disc, and moreparticularly relates to a wavefront aberration correcting device inwhich a liquid crystal is utilized, and an optical pickup devicecomprising this wavefront aberration correcting device,

BACKGROUND ART

It is well-known that information recording mediums for opticalinformation recording or information reading include optical discs suchas CD (compact disc) and DVD (digital versatile disc). A various opticaldiscs such as read-only optical discs, recordable optical discs in whichthe post recording of information is possible, and rewritable opticaldiscs on which the deleting and re-recording of information is possiblehave been developed.

Research and development into optical pickup devices and informationrecording and reading devices has proceeded alongside the high densityand increased capacity of these optical discs (for example, seenon-patent document 1). As a means for contending with optical discs ofhigh density as described above, the irradiation of a light beam ofsmall irradiation diameter on the optical disc by increasing thenumerical aperture (hereinafter referred to as “NA”) of an objectivelens provided in the optical pickup device has been considered. As anadditional means to contend with this increased densification,utilization of a short-wavelength light beam, or more particularly alight beam produced by a blue semiconductor laser, has been examined.

However, there is a problem inherent to the increasing of the NA of theobjective lens and the utilization of the short-wavelength light beam inthat there is an accompanying increase in the influence of aberration onthe light beam produced by the optical disc and, accordingly, theimprovement of information recording and information reading accuracybecomes difficult.

More specifically, if the optical disc tilts during the process ofinformation recording or information reading and the incident angle ofthe light tilts with respect to the normal direction of the optical disc(so called tilt angle), the comma aberration effect increases, Inaddition, although the extent of aberration changes depending on athickness of the optical disc substrate, there are problems inherent tothe reading of a CD using an optical pickup device for the reading of aDVD in which the optical disc thickness of the CD is greater than thatof the DVD in that this results in the generation of sphericalaberration and a significant expansion of the spot diameter of thelight.

On the other hand, in high-density optical discs such as DVD that havebeen developed in recent years, much attention has focused on the use ofa liquid crystal device as an aberration correcting element of theoptical system. The liquid crystal element, which is inserted into theoptical path of an optical pickup comprising a laser light source and anobjective lens, corrects the phase turbulence, that is to say, the lightwavefront turbulence that is caused by comma aberration that has itsorigin in the tilting of the optical disc substrate and by the wavefrontaberration generated when a multilayer disc substrate is read. Thisconstitutes a method in which the phase is controlled by changing of thevoltage applied to the liquid crystal.

By way of example of an liquid crystal device, as shown in FIG. 1, aliquid crystal device for modulating the phase of a monochromatic lightor a laser light has been disclosed in which a pair of opposingtransparent substrates on which an ITO layer which constitutes an oxideof indium and tin (hereinafter referred to simply as “ITO”.) and analignment film layer are laminated in sequence, a liquid crystal layeris provided between the alignment film layers, and an insulating film isformed between the alignment film layer and the ITO layer on at leastone of the transparent substrates, and in which the relationship betweenthe film thickness of the layers and the refractive indices thereof isoptimized to minimize the light transmittance fluctuations of the liquidcrystal device (for example, see patent document 1).

In addition, a liquid crystal device for correcting wavefront aberrationgenerated in an optical system appropriately without effect of a ¼ waveplate arranged in the optical path that leads from a light sourcethrough a recording medium to a light detector has been disclosed (forexample, see patent document 2). Although the technique disclosed forthe liquid crystal device of patent document 2 involves the use of acurved liquid crystal substrate, this curved shape can in no way beregarded as contributing to improving the light transmittance.

[non-patent document 1] Journal of the Japan Society of MechanicalEngineers 2001.4 Vol, 104 No, 989

[patent document 1] Japanese Laid-Open Patent Publication No.2002-208158

[patent document 2] Japanese Laid-Open Patent Publication No.2002-251774

DISCLOSURE OF INVENTION

However, the liquid crystal device shown in FIG. 1 (1) comprises amultilayer thin-film structure and, in addition, the effectiverefractive index of the liquid crystal layer, which is a constituentelement of the thin film, changes according to the drive of the liquidcrystal. As a result, a problem of light transmittance fluctuationexists. This is because the characteristic of a resonator is produceddue to the reflection at the interfaces between the films of themultilayer film. In particular, reflection from the interfaces with ITOfilms that have high refractive index is large and is surmised to be themain cause of light transmittance fluctuations.

In addition, in the liquid crystal element shown in FIG. 1, lighttransmittance fluctuations are suppressed by optimizing the refractiveindices and the film thickness of the transparent electrode films oralignment film from which the element is constituted. However, (2) theoptimum film thickness at the wavelength range of blue light (wavelengthof the order of approximately 400 nm) is comparatively thicker than atthe wavelength range of red light (wavelength of the order ofapproximately 650 nm) and, as a result, a problem of increasedabsorption loss arises.

In particular, the light transmittance in optical devices for laseroptical systems in which that laser power is limited such as opticalpickup devices is an essential element in the practical application ofthe optical pickup device. In the future, when the blue semiconductorlaser is used as the light source to develop high-density recording, theproblems with respect to the light transmittance fluctuation of theliquid crystal device will present a significant obstacle to thepractical application thereof and, coupled with the development of theshorter wavelength light source, there is high likelihood that theproblems outlined above will be further magnified.

Accordingly, there is a demand for, as a phase modulating device, aliquid crystal device of high light transmittance in which lighttransmittance fluctuations are reduced.

Therefore, and in view of the abovementioned problems of (1) and (2), afirst object of the present invention is the provision of a wavefrontaberration correcting device comprising a liquid crystal device thatminimizes light transmittance fluctuations and that has a high lighttransmittance.

In addition, a second object of the present invention is the provisionof an optical pickup device comprising the abovementioned wavefrontaberration correcting device.

The first object is achieved by a wavefront aberration correcting devicefor correcting a wavefront aberration of light generated in an opticalpath of an optical system for irradiating light onto a recording mediumor guiding reflected light reflected by the recording medium, the devicecomprising: a pair of opposing transparent electrode layers provided inthe optical path; and a liquid crystal sandwiched between thetransparent electrode layers, the liquid crystal generating phase changein passing light due to voltage applied to the transparent electrodelayers, wherein at least one of the transparent layers is arranged on anantireflective body comprising a substrate, and a finestructure which isformed on the substrate and which has a concave-convex structure.

In the preferred aspect of the wavefront aberration correcting deviceaccording to the present invention, the concave-convex structure isformed in a one-dimensional and/or a two-dimensional shape.

In the preferred aspect of the wavefront aberration correcting deviceaccording to the present invention, when the concave-convex structurehas a structure of changing periodically, a pitch of the concave-convexstructure is no more than 500 nm.

In the preferred aspect of the wavefront aberration correcting deviceaccording to the present invention, the antireflective body comprisesthe substrate and the finestructure that are both formed from either aglass or a resin, and the substrate and the finestructure are integrallyformed.

In the preferred aspect of the wavefront aberration correcting deviceaccording to the present invention, the antireflective body comprisesthe substrate formed from a glass and the finestructure formed from aresin.

In the preferred aspect of the wavefront aberration correcting deviceaccording to the present invention, the antireflective body comprisesthe substrate formed from a resin and the finestructure formed from aglass.

In the preferred aspect of the wavefront aberration correcting deviceaccording to the present invention, the wavefront aberration correctingdevice according to the present invention further comprises an alignmentfilm provided between the transparent electrode layer and the liquidcrystal.

In the preferred aspect of the wavefront aberration correcting deviceaccording to the present invention, the transparent electrode layercomprises an ITO layer that is an oxide of indium and tin.

In the preferred aspect of the wavefront aberration correcting deviceaccording to the present invention, the transparent electrode layer ispartitioned into pixels.

In the preferred aspect of the wavefront aberration correcting deviceaccording to the present invention, the light is a blue semiconductorlaser beam.

The second object is achieved by an optical pickup device comprising alight source that emits light for irradiation onto a recording medium,and an objective lens arranged between the light source and therecording medium, the objective lens converging the light from the lightsource onto an information recording surface of the recording medium,the optical pickup device comprising: a wavefront aberration correctingdevice arranged between the light source and the objective lens, thewavefront aberration correcting device comprising a pair of opposingtransparent electrode layers provided in an optical path in the opticalpickup device; and a liquid crystal sandwiched between the transparentelectrode layers, the liquid crystal generating phase change in passinglight due to voltage applied to the transparent electrode layers,wherein at least one of the transparent layer is arranged on anantireflective body comprising a substrate, and a finestructure which isformed on the substrate and which has a concave-convex structure.

In the preferred aspect of the optical pickup device according to thepresent invention, the concave-convex sturcture is formed in aone-dimensional and/or a two-dimensional shape.

In the preferred aspect of the optical pickup device according to thepresent invention, when the concave-convex structure has a structure ofchanging periodically, a pitch width of the concave-convex structure isno more than 500 nm.

In the preferred aspect of the optical pickup device according to thepresent invention, the antireflective body comprises the substrate andthe finestructure that are both formed from either a glass or a resin,the substrate and the finestructure are integrally formed.

In the preferred aspect of the optical pickup device according to thepresent invention, the antireflective body comprise the substrate formedfrom a glass and the finestructure formed from a resin.

In the preferred aspect of the optical pickup device according to thepresent invention, the antireflective body comprises the substrateformed from a resin and the finestructure formed from a glass.

In the preferred aspect of the optical pickup device according to thepresent invention, the optical pickup device according to the presentinvention further comprises an alignment film provided between thetransparent electrode layer and the liquid crystal.

In the preferred aspect of the optical pickup device according to thepresent invention, the transparent electrode layer comprises an ITOlayer that is an oxide of indium and tin.

In the preferred aspect of the optical pickup device according to thepresent invention, the transparent electrode layer is partitioned intopixels.

In the preferred aspect of the optical pickup device according to thepresent invention, the light is a blue semiconductor laser beam.

It should be noted that, although the term “finestructure” used hereinrefers to a structural body in which a structure of nanometer (onehundred-millionth of one meter) level is provided on the surfacethereof, the structure is in no way restricted to the nanometer leveland the term encompasses structural bodies of a structure of a 1 μmlevel.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view for explaining simply a liquid crystal device thatserves as a phase modulating device in the prior art;

FIG. 2 is a schematic cross-section view for explaining simply thewavefront aberration correcting device according to the presentinvention;

FIG. 3 is a perspective view of one embodiment of the shape of thefinestructure according to the present invention;

FIG. 4 is a perspective view of another embodiment of the shape of thefinestructure pertaining to the present invention;

FIG. 5 is a schematic view of one embodiment of the pixel-partitionedtransparent electrodes used in the present invention;

FIG. 6 is a cross-sectional diagram of the steps for explaining onemethod for manufacturing an antireflective body according to the presentinvention;

FIG. 7 is a cross-sectional diagram of the steps for explaining anothermethod for manufacturing the antireflective body according to thepresent invention. It should be noted that the arrows of FIGS. 7( b) and7(c) indicate an irradiated electron beam whose dose amount iscontrolled;

FIG. 8 is a cross-sectional diagram of the steps for explaining afurther method for manufacturing the antireflective body according tothe present invention;

FIG. 9 is a schematic diagram for explaining the constitution of oneembodiment of the optical pickup device comprising the wavefrontaberration correcting device according to the present invention;

FIG. 10 is a diagram illustrating the light transmittance simulationresults of a liquid crystal device comprising the antireflective bodywith a finestructure according to the present invention, in which withthe finestructure surface of a periodic concave-convex structure, thepitch of the periodic concave-convex structure was changed. It should benoted that this simulation was implemented using a light of wavelength400 nm;

FIG. 11 is a diagram illustrating the light transmittance simulationresults of a liquid crystal device comprising the antireflective bodywith a finestructure according to the present invention, in which withthe finestructure surface of a periodic concave-convex structure, thedepth of the periodic concave-convex structure was changed. It should benoted that this simulation was implemented using a light of wavelength400 nm;

FIG. 12 is a diagram showing the light transmittance wavelengthdependency simulation results in a comparison between the liquid crystaldevice comprising the antireflective body according to the presentinvention and a liquid crystal device not comprising the antireflectivebody;

FIG. 13 is a diagram illustrating the light transmittance simulationresults of a liquid crystal device comprising the antireflective bodyaccording to the present invention, in which with an antireflective bodysurface of a periodic concave-convex structure, the pitch of theperiodic concave-convex structure was changed. It should be noted thatthis simulation was implemented using a light of wavelength 650 nm;

FIG. 14 is a diagram illustrating the light transmittance simulationresults of a liquid crystal device comprising the antireflective bodyaccording to the present invention, in which with an antireflective bodysurface of a periodic concave-convex structure, the depth of theperiodic concave-convex structure was changed. It should be noted thatthis simulation was implemented using a light of wavelength 780 nm;

FIG. 15 is a diagram illustrating the light transmittance simulationresults (TM) of the liquid crystal device comprising the antireflectivebody according to the present invention as a whole in which the phase ofthe liquid crystal layer was changed by an applied voltage; and

FIG. 16 is a diagram showing the light transmittance simulation results(TE) of the liquid crystal device comprising the antireflective bodyaccording to the present invention as a whole in which the phase of theliquid crystal layer was changed by an applied voltage.

BEST MODE FOR CARRYING OUT THE INVENTION

A description is given below of an embodiment of the wavefrontaberration correcting device according to the present invention and theoptical pickup device comprising the same. It should be noted thatalthough the embodiment described below represents a specificallysuitable example of the present invention to which the preferredtechnical restrictions are imposed, there are no particular restrictionsto the present invention in the description thereof, and the scope ofthe present invention should not be regarded as restricted to this mode.

FIG. 2 is a cross-section view of the constitution of a wavefrontaberration correcting device 100 according to the present invention. Asis shown in the view, a liquid crystal molecule is sealed in thewavefront aberration correcting device 100 by a pair of substrates 110according to the present invention. The substrate material of thepresent invention may be the same as or different from the material of alater-described finestructure 120. Although there are no particularrestrictions thereto, a glass and a transparent resin are specificexamples of preferred substrate materials. Specific examples of theresin used include polymethyl methacrylate and polycarbonate.

Transparent electrode films 130 for the application of a voltage to theliquid crystal 150 by means of a liquid crystal drive device 200 arearranged on opposing surfaces of the substrates 110. Furthermore,alignment films 140 are formed on the inner side of the electrode films130. Specific examples of the transparent electrode films 130 used inthe present invention include a zinc oxide-based transparent electrodefilm, an indium oxide-zinc oxide-based transparent electrode film, and atin-added indium oxide transparent electrode film. Considering lighttransmissivity, an ITO (hereinafter referred to simply as “ITO” layer),which constitutes an oxide of indium and tin, is preferred.

It should be noted that, although there are no particular restrictionsto the liquid crystal molecule used in the present invention, as long asthe liquid crystal molecule has an electrooptic effect by which theorientation thereof changes due to applied voltage generating phasechanges in passing light, an homogenous-type liquid crystal or anhomeotropic-type liquid crystal in which the crystals are aligned eitherin parallel or perpendicular to the substrate is preferred.

The ITO film 130 of the wavefront aberration correcting device 100according to the present invention is arranged on a finestructure 120 onwhich a one-dimensional and/or a two-dimensional concave-convexstructure is provided on the surface thereof.

FIG. 3 is a perspective view of one embodiment of the shape of thefinestructure 120 according to the present invention. The finestructure120, as is shown in FIG. 3, is formed on the substrate 110, and anantireflective body 180 comprises the substrate 110 and thefinestructure 120. Although the substrate 110 and the finestructure 120are illustrated separately in FIG. 3, if the substrate 110 and thefinestructure 120 form from an identical material, an integratedantireflective body 180 may be formed. Although the shape of thefinestructure 120 shown in FIG. 3 is formed in a periodicone-dimensional shape, it is not necessarily the case that thisone-dimensional shape of the concave-convex structure must have aperiodic formation.

FIG. 4 is a schematic perspective view of the finestructure of the shapeof another embodiment according to the present invention. Although theshape of the finestructure 120 illustrated in FIG. 4 is formed in aperiodic two-dimensional shape, in the same way as the description ofFIG. 3, it is not necessarily the case that this two-dimensional shapeof the concave-convex structure must have a periodic formation.

Furthermore, the object of the present invention can be achieved withthe finestructure according to the present invention of a shape thatcombines both the one-dimensional concave-convex shape shown in FIG. 3and the two-dimensional concave-convex shape shown in FIG. 4.

It should be noted that the term “one-dimensional” used herein refers toa dimension in which the shape of the concave-convex structure on thesurface changes in one direction, on the other hand, the term“two-dimensional” used herein refers to a dimension in which the shapeof the concave-convex structure on the surface changes in twodirections.

By the formation of the ITO film 130 on the concave-convex structure bya method of sputtering or electron beam vacuum deposition or the like,the ITO film 130 itself is formed in a shape that is similar to theshape of the concave-convex structure. Accordingly, as described below,an improvement in light transmittance can be achieved based on theantireflection of light. It should be noted that, in view of the factthat the transparency of the film 130 itself should be high, it ispreferable for the transparent electrode film 130 to be formed in a filmthickness of the order of 10 to 50 nm.

FIG. 5 is a schematic view of one embodiment of the pixel-partitionedtransparent electrode film used in the present invention. As is shown inFIG. 5, the transparent electrode, which describes a concentric circleshape and is radially partitioned, is able to produce voltagedistribution for applying to the liquid crystal based on the applicationof voltage of a different level for each partitioned electrode film. Itshould be noted that the black section of the FIG. 5 indicates the ITOfilm 130.

Incidentally, when mediums of different refractive indices are laminatedthrough interfaces, reflection of light occurs at the interfaces becauseof the rapid change in the refractive index at these interfaces. On theother hand, the lamination of mediums of two types of differentrefractive indices through the interfaces with a concave-convex shapehas the effect of effectively changing the refractive indicescontinuously at the transition region thereof. However, the phenomenonof gradual and continuous change of a refractive index can occur whenthe period of the concave-convex structure is small by comparison withthe light wavelength. Utilizing this phenomenon, the antireflectioneffect according to the present invention can be provided by way of theantireflective body.

In particular, based on the later-described simulation results, althoughthe pitch of the concave and convex on the surface of the finestructure120 according to the present invention (see pitch width of FIG. 2) isdependent on the relationship between the refractive indices of the ITOfilm or the alignment film and the light wavelength, in a case where theconcave-convex structure has a periodically changing structure, it ispreferable that the pitch is no more than 500 nm and, more preferably,that it is no more than 350 nm and, more preferably, that it is no morethan 250 nm.

Although there are no restrictions to the material of the finestructureaccording to the present invention, from the viewpoint of lighttransmittance it is preferable that the material be a glass or atransparent resin.

Although a description is given next of a typical method formanufacturing the antireflective body 110 comprising the substrate 110and the finestructure 120, there are no particular restrictions to themethod for manufacturing the antireflective body 180 according to thepresent invention.

FIG. 6 is a cross-sectional view of the steps for explaining one methodfor manufacturing the antireflective body comprising the finestructureaccording to the present invention. The manufacturing method illustratedin FIG. 6 constitutes a method for manufacturing a finestructure basedon the repetition of a lithography process.

More specifically, as shown in FIG. 6( a), a resist film 420 ispre-coated on a glass substrate 430 in advance and is exposed to lightthrough a mask 410 having a first predetermined pattern produced inadvance and is developed to form a pattern on the glass substratecorresponding to the first predetermined pattern (see FIG. 6( b)).Thereafter, as illustrated in FIG. 6( c), a first pattern is formed onthe glass substrate by the etching of the glass using an ion beam. Next,a resist 450 is formed on the entire surface of the glass substrate (seeFIG. 6( d)). As shown in FIG. 6( e), a resist pattern 460 is formed onthe glass substrate 430 by exposure and development by way of a mask 440having a second pattern different from the first pattern. Next, theglass substrate is etched using an ion beam. By the repetition of theprocess described above for the number of times based on need, as shownin FIG. 6( f), an antireflective body 480 with a finestructurecomprising the concave-convex structure on its surface can bemanufactured.

FIG. 7 is a cross-sectional view of the steps for explaining anothermethod for manufacturing an antireflective body comprising thefinestructure according to the present invention. The manufacturingmethod shown in FIG. 7 constitutes a method that combines the use of alow γ resist and the control of the dose amount based on electron beamdrawing.

More specifically, as shown in FIG. 7( a), a low γ resist 510 is formedon a glass substrate 500. Then, the drawing of electron beam having acontrolled dose amount occurs in sequence from one end on the resist(see FIG. 7( b) to FIG. 7( e)). It should be noted that the arrows inFIGS. 7( b) and 7(c) indicate the irradiation of the controlled doseamount of electron beam. Thereafter development occurs forming a resistfilm of a predetermined pattern on the glass substrate (see FIG. 7( f)).As shown in FIG. 7( g), the antireflective body 580 comprising afinestructure with a concave-convex structure on the surface can bemanufactured by subsequent etching.

The antireflective bodies 480, 500 of the integrated substrate andfinestructure can be manufactured using the manufacturing methodsexplained with reference to FIG. 6 and FIG. 7, Furthermore, by attachinga plate-shaped body comprising a resin to the lower part of thefinestructure manufactured by the manufacturing methods depicted in FIG.6 and FIG. 7, an antireflective body of which the substrate comprisingthe resin and in which the finestructure comprising the glass can bemanufactured.

FIG. 8 is a cross-sectional view of the steps for the explaining afurther method for manufacturing an antireflective body comprising thefinestructure according to the present invention. The manufacturingmethod illustrated in FIG. 8 constitutes a method based on use of astamper comprising a prescribed concave-convex shaped structure on itssurface produced by electroforming, that is, a transfer of a shape ofthe stamper to the resin.

More specifically, as shown in FIG. 8( a), a mould 600 produced bycarrying out a conductive processing on a substrate surface comprising afine pattern is electroformed with nickel (see reference symbol 610 ofFIG. 8( b)), whereupon the mould 600 is separated or dissolved from thesubstrate producing a stamper 630 with a concave-convex shaped surface(see FIG. 8( c)). As shown in FIG. 8( d), by pressing the stamper 630onto a transparent resin 640, the surface shape of the stamper 630 istransferred to the transparent resin 640 to manufacture anantireflective body 680 comprising a concave-convex structure on itssurface. It should be noted that, in the implementation of transferusing the stamper, the ease and speed of this transfer is promoted byheating the resin.

An antireflective body comprising a glass substrate and resin can bemanufactured by attaching the glass substrate to the lower part of theresin 640 transferred in this way.

The finestructure according to the present invention can be manufacturedby the abovementioned manufacturing methods, and the shape and size ofthe concave-convex structure of the surface of the finestructure or,more specifically, the pitch of this concave-convex structure, can bemanufactured in any size, as required, by the control of the maskpattern and electron beam spot diameter and, furthermore, by the controlof the precision of the stamper. More specifically, from the viewpointof antireflection, it is preferable that the pitch of the periodicstructure of the concave-convex structure of the surface of thefinestructure according to the present invention be smaller than thelight wavelength utilized for information reading of the recordingmedium as described above.

FIG. 9 is a schematic explanatory diagram of the constitution of oneembodiment of the optical system of an optical pickup device 700comprising the wavefront aberration correcting device 100 according tothe present invention. A collimator lens 720, a beam splitter 730 andthe wavefront aberration correcting device 100 according to the presentinvention able to be controlled by the liquid crystal driver 200 areprovided in sequence along an optical axis OA in an optical path betweena laser light source 710, that serves as the light source 710, and anobjective lens 740.

It should be noted that the laser light source of the present inventionutilizes a blue laser of wavelength of the order of 400 nm from a nearinfrared region laser of wavelength 780 nm or red laser of wavelength650 nm. In particular, by combining a blue laser which has lowerlight-emitting efficiency than a red laser, the wavefront aberrationcorrecting device according to the present invention is able to maximizeits light utilization efficiency thereof.

The optical beam irradiated from the laser light source 710 of theoptical pickup device 700 is led to the objective lens 740 through thecollimator lens 720, the beam splitter 730 and wavefront aberrationcorrecting device 100. The light beam is converged by the objective lens740 and focused on the information recording surface of the optical disc750.

The optical beam emitted from the light source 710 is reflected by theoptical disc 750 and the return path optical beam reflected by disc 750is divided by the beam splitter 730, the divided beam is converged by aconverging lens 760 and imaged on and detected by the light detector770. When wavefront aberration is generated as a result of either thetilting of the substrate of the optical disc 750 or a change in thethickness of the optical disc 750, the light detector 770 sendswavefront aberration correcting signals 780 from the liquid crystaldriver 200 to the wavefront aberration correcting device 100 and, bycontrolling the orientation state of the liquid crystal molecules of thewavefront aberration correcting device 100, aberration correction issuccessively implemented.

EMBODIMENT

The description given below illustrates simulation results of the lightreflectance characteristics of the liquid crystal device comprising theantireflective body comprising the finestructure according to thepresent invention. However, the technical scope of the present inventionis in no way restricted to the simulation results outlined below.

There are no particular restrictions to the liquid crystal devicecomprising the antireflective body according to the present inventionemployed in the simulations outlined below which comprises, in order, asubstrate (Corning #1737, ultraviolet cured methacrylic resinmanufactured by Mitsubishi Chemical (Corp.): refractive index 1.52), ITOlayer (refractive index 2.0, film thickness 30 nm), alignment film(refractive index 1.75, film thickness 30 nm), and liquid crystal layer(Ne=1.85). The antireflective body according to the present inventionrefers to a material comprising a substrate and a finestructureconstituted in such a way that a concave-convex structure formed on thesubstrate forms the surface thereof.

In the calculation conditions for these simulations the pitch of theperiodic concave-convex structure of the substrate surface was taken asbetween 10 to 350 nm, and the depth of the periodic concave-convexstructure was taken as 0.75 times the abovementioned pitch. Here, thepitch of the periodic concave-convex structure refers to the pitchillustrated in FIG. 2, and the depth of the periodic concave-convexstructure refers to the distance between the apex part of the projectingpart and the base part of the recess part thereof. In addition, thesimulations were implemented on the presumption that light of wavelength400 nm was used.

FIG. 10 is a diagram showing the light transmittance simulation resultsof the liquid crystal device comprising the antireflective bodyaccording to the present invention in which the pitch of the periodicconcave-convex structure was changed. Here, TE is an abbreviation fortransverse electronics and TM is an abbreviation for the transversemagnetics that represent the polarized state of incident light. As seenin the results shown in FIG. 10, the value of TM begins to decrease at apitch of greater than 220 nm and the light transmittance at pitch of 350nm was approximately 10% less than the light transmittance at pitch of200 nm. The decrease in light transmittance in the region of theperiodic concave-convex structure of pitch of greater than 250 nm wasobserved, which is caused by diffraction phenomena.

Next, taking the pitch of the periodic concave-convex structure as 200nm and, except for the changing of the depth of the periodicconcave-convex structure between 5 to 300 nm, simulation was implementedunder the conditions identical to that described above. FIG. 11 is adiagram that illustrates the light transmittance simulation results of aliquid crystal device comprising the antireflective body according tothe present invention in which the depth of the periodic concave-convexstructure was changed. As is clear from the results of FIG. 11, using aliquid crystal device that utilizes the antireflective body according tothe present invention, simulation results in which there was littlechange in light transmittance at the periodic concave-convex structuredepth of 100 nm or more of the surface of the finestructure from whichthe antireflective body is formed and in which the light transmittancewas essentially fixed were obtained.

FIG. 12 is a diagram that illustrates the light transmittance simulationresults of wavelength dependency in a comparison between the liquidcrystal device comprising the antireflective body according to thepresent invention and a liquid crystal device that does not comprise theantireflective body. It should be noted that the pitch of the periodicconcave-convex structure of the surface of the antireflective body wasset as 200 nm and the depth thereof was set as 150 nm and, except forthe pitch and the depth, the simulation conditions adopted wereidentical to those of the simulation as shown in FIG. 10. In addition,in view of the fact that the emission wavelength of semiconductor lasersutilized for optical discs and the like fluctuates in accordance withthe environmental temperatures during operation, the simulation for theexamination of wavelength dependency was implemented using a bluesemiconductor laser at an approximate wavelength having a few nanometerrange centered in 400 nm. Furthermore, the “no structure” of FIG. 12refers to light transmittance simulation results for a liquid crystaldevice not comprising an antireflective body having a periodicconcave-convex structure on its surface.

As is clear from the results of FIG. 12, using the liquid crystal devicecomprising the antireflective body according to the present invention,simulation results of high light transmittance were obtained in theregion of the emission wavelength of a blue semiconductor laser duringnormal operation in a polarized state of TE and TM light. With the “nostructure” of FIG. 12, the light transmittance simulation results for TEand TM were indistinguishable due to the perpendicular reflection oflight.

Next, using light of wavelength 650 nm that is employed in optical discssuch as CD or CD-R, simulation of the light transmittance of the liquidcrystal device comprising the antireflective body according to thepresent invention was implemented with respect to the pitch of theantireflective body, and the results thereof are shown in FIG. 13. Itshould be noted that, in the simulation conditions shown in FIG. 13, thepitch of the periodic concave-convex structure of the surface of theantireflective body was taken as between 5 to 800 nm, and the depth ofthe periodic concave-convex structure was taken as 0.75 times theabovementioned pitch.

FIG. 14 shows the simulation results in which, under the conditionsidentical to the simulation conditions shown in FIG. 13, light ofwavelength 780 nm was used.

It is clear from the results shown in FIG. 13 that high lighttransmittance for both TE and TM were obtained at pitch of no greaterthan approximately 370 nm. In addition, it is clear from the resultsshown in FIG. 14 that high light transmittance for both TE and TM wereobtained at pitch of no greater than approximately 500 nm.

It is apparent from a comparison of the results shown in FIG. 10 withthe results shown in FIG. 13 and FIG. 14 that the pitch of the periodicconcave-convex structure for obtaining high light transmittance aredependent on the wavelength of the light utilized. Because the lighttransmittance of the liquid crystal device changes in accordance withthe refractive indices of the substrate, the transparent electrodes, thealignment film and the liquid crystal, the pitch of at least theperiodic concave-convex structure of the antireflective body surfacemust be no greater than 500 nm, in order to achieve high lighttransmittance using the liquid crystal device comprising theantireflective body according to the present invention.

A liquid crystal device produced by two ITO films, which constitutetransparent electrodes, arranged on the antireflective body according tothe present invention and the sandwiching of a liquid crystal layer(Ne=1.85) there-between was assumed, and a light transmittancesimulation of the device with respect to a later-described appliedvoltage was implemented.

The simulation outlined below was implemented under the conditions inwhich the pitch of periodic concave-convex structure of theantireflective body according to the present invention was taken as 200nm and the depth thereof was taken as 150 nm.

FIG. 15 is a diagram that illustrates the light transmittance simulationresults (TM) of the liquid crystal device where the phase difference ofthe liquid crystal layer was changed due to an applied voltage. Itshould be noted that the liquid crystal phase differences of FIG. 15 andthe later-described FIG. 16 refer to phase changes generated due tochanges in the refractive index of the liquid crystal layer produced bythe applied voltage.

It is clear from the simulation results of FIG. 15 that, even if therefractive index of the liquid crystal layer within the liquid crystaldevice is changed by the applied voltage, the light transmittancefluctuation rate of the device in the TM polarized state is no more than0.1%.

FIG. 16 is a diagram that illustrates the light transmittance simulationresults (TE) of the liquid crystal device as a whole where the phasedifference of the liquid crystal layer is changed by an applied voltage.As is clear from the simulation results shown in FIG. 16, the lighttransmittance fluctuation rate is no greater than 0.01% even when theapplied voltage is changed.

The above simulation results indicate that light transmittancefluctuation with respect to applied voltage is small in a wavefrontaberration correcting device in which a structure in which theantireflective body according to the present invention is arranged onthe transparent electrodes is adopted. This is because, based on thefact that the reflectance of the ITO film that constitutes a transparentelectrode that plays the role of a mirror of a resonator structure whichis a cause of light transmittance fluctuations can be reduced, thefunction of the resonator is impaired.

A structure comprising the sandwiching of the liquid crystal between ITOfilms can be utilized even in liquid crystal display devices.Accordingly, the wavefront aberration correcting device according to thepresent invention is not confined to application as a laser wavefrontcontrol device and can have application in liquid crystal devices or,more particularly, in reflection-type color liquid crystal displaydevices. In addition, because of the elimination of the characteristicsas a resonator as described above and, as a natural outcome, because ofthe reduction in transmittance fluctuations depending on thewavelengths, a display device for which the setting of color balance issimple, and which displays good color reproductivity, can be provided.

INDUSTRIAL APPLICABILITY

According to the present invention, in a wavefront aberration correctingdevice comprising a liquid crystal device comprising a liquid crystalmolecule sandwiched between two substrates, because the light reflectionwithin the abovementioned device can be efficiently prevented by theprovision of an antireflective body with a finestructure on theabovementioned substrate and the arrangement of transparent electrodeson the antireflective body, an improvement in the light transmittance ofthe wavefront aberration correcting device as a whole is achieved. Inaddition, light transmittance fluctuations of the wavefront aberrationcorrecting device accompanying wavefront aberration correction can besuppressed to a minimum. In addition, not only is the wavefrontaberration correcting device comprising the antireflective bodyaccording to the present invention able to correct aberration that hasits origins in the change in the substrate thickness and tilting of anoptical disc, it is also correct aberration in the optical system itselfsuch as in the converging lens.

Furthermore, in optical pick up devices comprising the wavefrontaberration correcting device according to the present invention, becausethe light transmittance of the abovementioned wavefront aberrationcorrecting device is high particularly when assembled together with ablue semiconductor laser for emitting light of a short wavelength, anoptical pickup device of high light utilization efficiency can beprovided,

1. A wavefront aberration correcting device for correcting a wavefrontaberration of light generated in an optical path of an optical systemfor irradiating light onto a recording medium or guiding reflected lightreflected by the recording medium, the device comprising: a pair ofopposing transparent electrode layers provided in the optical path; anda liquid crystal sandwiched between the transparent electrode layers,the liquid crystal generating phase change in passing light due tovoltage applied to the transparent electrode layers, wherein at leastone of the transparent layers is arranged on an antireflective bodycomprising a substrate, and a finestructure which is formed on thesubstrate and which has a concave-convex structure.
 2. The wavefrontaberration correcting device according to claim 1, wherein theconcave-convex structure is formed in a one-dimensional and/or atwo-dimensional shape.
 3. The wavefront aberration correcting deviceaccording to claim 1 or 2, wherein when the concave-convex structure hasa periodically changing structure, a pitch of the concave-convexstructure is no more than 500 nm.
 4. The wavefront aberration correctingdevice according to claim 1 or 2, wherein the antireflective bodycomprises the substrate and the finestructure that are both formed fromeither a glass or a resin, and the substrate and the finestructure areintegrally formed.
 5. The wavefront aberration correcting deviceaccording to claim 1 or 2, wherein the antireflective body comprises thesubstrate formed from a glass and the finestructure formed from a resin.6. The wavefront aberration correcting device according to claim 1 or 2,wherein the antireflective body comprises the substrate formed from aresin and the finestructure formed from a glass.
 7. The wavefrontaberration correcting device according to claim 1 or 2, furthercomprising an alignment film provided between the transparent electrodelayer and the liquid crystal.
 8. The wavefront aberration correctingdevice according to claim 1 or 2, wherein the transparent electrodelayer comprises an ITO layer that is an oxide of indium and tin.
 9. Thewavefront aberration correcting device according to claim 1 or 2,wherein the transparent electrode layer is partitioned into pixels. 10.The wavefront aberration correcting device according to claim 1 or 2,wherein the light is a blue semiconductor laser beam.
 11. An opticalpickup device comprising a light source that emits light for irradiationonto a recording medium, and an objective lens arranged between thelight source and the recording medium, the objective lens converging thelight from the light source onto an information recording surface of therecording medium, the optical pickup device comprising: a wavefrontaberration correcting device arranged between the light source and theobjective lens, the wavefront aberration correcting device comprising apair of opposing transparent electrode layers provided in an opticalpath in the optical pickup device; and a liquid crystal sandwichedbetween the transparent electrode layers, the liquid crystal generatingphase change in passing light due to voltage applied to the transparentelectrode layers, wherein at least one of the transparent electrodelayer is arranged on an antireflective body comprising a substrate, anda finestructure which is formed on the substrate and which has aconcave-convex structure.
 12. The optical pickup device according toclaim 11, wherein the concave-convex structure is formed in aone-dimensional and/or a two-dimensional shape.
 13. The optical pickupdevice according to claim 11 or 12, wherein when the concave-convexstructure has a periodically changing structure, a pitch of theconcave-convex structure is no more than 500 nm.
 14. The optical pickupdevice according to claim 11 or 12, wherein the antireflective bodycomprises the substrate and the finestructure that are both formed fromeither a glass or a resin, the substrate and the finestructure areintegrally formed.
 15. The optical pickup device according to claim 11or 12, wherein the antireflective body comprise the substrate formedfrom a glass and the finestructure formed from a resin.
 16. The opticalpickup device according to claim 11 or 12, wherein the antireflectivebody comprises the substrate formed from a resin and the finestructureformed from a glass.
 17. The optical pickup device according to claim 11or 12, further comprising an alignment film provided between thetransparent electrode layer and the liquid crystal.
 18. The opticalpickup device according to claim 11 or 12, wherein the transparentelectrode layer comprises an ITO layer that is an oxide of indium andtin.
 19. The optical pickup device according to claim 11 or 12, whereinthe transparent electrode layer is partitioned into pixels.
 20. Theoptical pickup device according to claim 11 or 12, wherein the light isa blue semiconductor laser beam.